1. Field of the Invention
This invention relates generally to the field of semiconductor processing and, more particularly, to thermal reactors used for thermal processing.
2. Description of the Related Art
Semiconductor processing in a thermal reactor typically occurs in the reaction chamber of the thermal reactor, where semiconductor substrates loaded inside the reaction chamber are typically exposed to process gases, which typically chemically react with the substrates. For example, in thermal oxidation processes, the substrates are typically exposed to process gases such as O2 gas or to water vapor or to a mixture of O2 and water vapor to oxidize the substrates.
Before and after the exposure of the substrates to the process gas(es), the reaction chamber is generally purged with an inactive purging gas which does not react with the substrate. This purging is typically performed to avoid any undesirable reactions before and after the processing step, during which reactive process gases are flowed into the chamber. A purge typically removes any reactants from the chamber before the processing step and also removes the process gases and any byproduct from the reaction chamber after the processing step. Thus, by purging and flowing process gases into a chamber, the gas atmosphere in the process chamber can be changed several times during a process cycle.
It has been observed that this change of atmosphere can introduce undesirable transition effects, as described in U.S. Pat. No. 6,316,371, METHOD FOR THE CHEMICAL TREATMENT OF A SEMICONDUCTOR SUBSTRATE, issued Nov. 13, 2001 and assigned to ASM International, N.V., the entire disclosure of which is incorporated herein by reference. That patent describes a vertical thermal oxidation furnace which operates at substantially atmospheric pressure. In such a furnace the gas is typically introduced at the top of the furnace and is exhausted at the bottom, causing gas inside the reaction chamber to flow in a generally downward direction.
While the process results across a wafer and between individual wafers are preferably uniform, it has been observed that transitioning from one gas to another in a reaction chamber can detrimentally affect the uniformity of process results.
Accordingly, there is a need for methods for effectively removing gases and changing atmospheres in a reaction chamber, while minimizing the impact of atmosphere changes on process results.